Lead has been known to be a toxic element for more than a century. In recent decades, scientists and the public have become aware of the harm lead does to children, producing severe adverse effects to their mental and behavioral abilities in direct proportion to the concentration level of lead in their bodies. Governments have regulated the maximum concentration of lead in paint and in the bulk materials of consumer goods in order to protect the safety of the public, prevent the build up of toxic elements in waste disposals, and prevent its reuse in recycling.
In the Consumer Product Safety Improvement Act of 2008 (CPSIA), signed into law Aug. 14, 2008, the U.S. regulations were revised significantly. The older regulations required that the concentration of lead, in either the bulk of the material or the paint on the material, can be no greater than 600 parts per million; that is, that there must not be more than 600 μg of lead in each gram of either the bulk material or the paint on it. The new regulations have reduced the weight concentration of either bulk or paint to 90 ppm. The regulations allow the examination of small areas of paint (less than 10 milligram (mg) in weight) to be examined by x-ray fluorescence to meet a requirement that the areal density of the lead, that is, the weight in a square centimeter of paint (alternatively referred to as the mass thickness), be no greater than 2 μg/cm2. The CPSIA was a result of public demand for better consumer safety after the discovery that a significant number of toys had levels of lead that far exceeded the previously existing regulations.
The challenge for maintaining public safety is how to enforce the regulations when most of the billions of the toys and consumer products are imported from countries that have minimal supervision of the production of the bulk material or the paint. Making the problem even more difficult is that the new regulations continue to be based on the weight concentration of the lead, which is an appropriate measure for raw material but perhaps not for finished goods. The measurement of the weight concentration of the lead is by definition the measurement of the weight of the lead in a known weight of the bulk or the paint. Measuring the weight percent of lead in paint on a toy, for example, requires scraping off a paint chip, measuring its weight and then measuring the lead concentration at a certified laboratory using such techniques as inductively-coupled plasma optical emission spectroscopy (ICP-OES), inductively-coupled plasma mass spectrometry (ICP-MS), or flame atomic emission spectroscopy. These techniques are generally destructive, time-consuming and expensive.
Several references in the prior art (see, e.g., U.S. Patent Application Publication No. 2009/0067572 by Grodzins et al.) have demonstrated the ability of XRF instruments to distinguish between surface lead (i.e., lead present primarily in the paint or other coating layer) and bulk lead (i.e., lead distributed through the bulk thickness). Once the location of the lead is identified, its concentration may be calculated using location-specific algorithms. However, accurate calculation of the lead content by the disclosed techniques may require prior knowledge of the construction of the inspected object and/or prolonged irradiation/analysis times. For testing of consumer products, such as toys, information regarding the construction of the object to be inspected may not be available to the operator, potentially compromising the accuracy of the measurements. Furthermore, it is generally desirable to minimize the required time for analysis in order to increase testing throughput and reduce overall costs. Against this background, there is a need for an accurate lead concentration testing method that may be administered relatively quickly and inexpensively.